An ideal regulator circuit generates a constant output voltage or current for use by a particular circuit or application, despite changes in the load placed on the regulator and/or changes in an input voltage, current, temperature, noise, and other variables. Real-world regulators, however, are not ideal, and changes to some or all of these conditions produce undesirable changes in the output voltage or current.
One type of voltage regulator uses a DC-to-DC converter called an inverted or “floating” buck converter to step down a larger input DC voltage to a smaller output DC voltage. Inverted buck converters may be used, in particular, in applications that do not require a grounded load, such as LED lighting systems. FIG. 1 illustrates a circuit diagram of an exemplary inverted buck regulator 100. An input voltage Vin is applied to a terminal of a load RL and a load current ID flows through the load RL, thereby developing a load voltage VL. The load current ID also flows through an inductor L and a switch S. The opening and closing of the switch S is controlled by a pulse-width modulation circuit 102; the duty cycle D of the PWM pulses 104 may be varied to maintain a constant output voltage VO. The output voltage VL and/or the output current ID are sampled by a voltage/current measurement circuit 106; a controller 108 receives a sensor signal 110 and adjusts the PWM circuit 102 accordingly.
The inductor L serves to keep the current through the load constant and the capacitor tries to keep the voltage across the load constant. Thus a high-frequency rectangular-shaped PWM waveform with a certain duty cycle D is filtered by a second-order filter (L+C) to extract the DC value for use by the load. Since there are two energy-storage elements (L stores magnetic energy and C stores electrical energy), the system is second-order.
One problem with the circuit 100 of FIG. 1, however, is that it is susceptible to variations in the input voltage; the output current, for example, may undesirably vary if the input voltage varies. Some existing systems measure the inductor current ID directly, attempt to detect any changes in it, and modify the PWM pulses to reduce the magnitude of the changes. Other systems may measure the inductor current ID indirectly (by, for example, converting it to a voltage via the use of a small current-sense resistor in series with the load RL). Each of these measurement techniques, however, requires complex, precise, and/or expensive circuitry; the voltage across the current-sense resistor, for example, may be very small and require very precise measurement. Furthermore, existing systems may only reduce, not eliminate, changes in the output current ID produced by a change in Vin (for example), and this reduction may be insufficient to meet the needs of a given consumer of the produced output current. Finally, because existing systems only react to changes in the output current ID after they occur, the time it takes to detect and correct for these changes may be unacceptable. A need therefore exists for a regulator that produces an output current (or voltage) that is independent of any changes in an input voltage (or current).